319 questions from 36 interviews · updated from reports 2016-2025
About
American Express is a financial services company best known for credit cards, charge cards, payments, and travel-related services. In India, it hires technical talent for technology internships, software development engineer roles, and other intern positions in engineering and data teams.
The roles that come up most are Technology Intern, Intern and Software Development Engineer. This covers 36 candidate interviews reported from 2016 to 2025. Most sat it at internship level (21 of 36 that recorded a level). Among the 30 that recorded either route, arrivals split between campus drives (23, 77%) and off-campus applications (7, 23%). Most questions fall under CS fundamentals and DSA.
Ans. Use a greedy max-heap of the remaining counts for a, b and c. Repeatedly choose the character with the highest remaining count, unless it would create three consecutive equal characters; in that case choose the next highest. If no valid character exists, stop. Time complexity is O(n log 3), effectively O(n).
Ans. ACID properties are the guarantees that make database transactions reliable: Atomicity, Consistency, Isolation and Durability. Atomicity means all or nothing, Consistency keeps valid rules, Isolation prevents concurrent transactions interfering, and Durability ensures committed changes survive crashes. They are essential for correctness in systems handling critical data.
Ans. C is a procedural language, while C++ is largely a superset of C with object oriented and generic programming features. C++ adds classes, inheritance, polymorphism, templates, exceptions, references, function overloading and the standard library. The most important difference is abstraction: C gives low level control, while C++ supports higher level design without losing that control.
Ans. Abstraction in Object-Oriented Programming is the idea of exposing only the essential features of an object while hiding unnecessary implementation details. The key detail is that it lets users work with what an object does, not how it does it, often through interfaces, abstract classes, and public methods.
Ans. SQL databases store structured data in tables with fixed schemas and use SQL for relational queries. NoSQL databases use more flexible models such as documents, key value pairs, columns, or graphs. The key difference is that SQL favours strong consistency and complex joins, while NoSQL often favours flexibility, scale, and high availability.
Ans. Inheritance is an object-oriented programming concept where a class derives properties and behaviour from another class, enabling code reuse and specialisation. The main types are single, multiple, multilevel, hierarchical and hybrid inheritance. The key detail is that language support differs, for example Java does not allow multiple inheritance of classes but supports it through interfaces.
Ans. Use a recursive depth first search: if the current root is null or equals either target node, return it. Search left and right subtrees. If both return non-null, the current root is the LCA; otherwise return the non-null side. This uses the call stack, with linear time and tree-height space.
Ans. Function overloading means defining multiple functions with the same name but different parameter lists, while function overriding means a subclass provides its own implementation of a method already defined in its superclass. Overloading is resolved at compile time in many languages; overriding is resolved at run time using dynamic dispatch.
Ans. RAID types combine disks for performance, redundancy, or both. RAID 0 stripes data for speed but has no fault tolerance. RAID 1 mirrors data and survives one disk failure. RAID 5 uses distributed parity and survives one failure. RAID 6 survives two failures. RAID 10 combines mirroring and striping for speed and resilience.
Ans. HTTP is a stateless request response protocol where a client, usually a browser, sends a request to a server and receives a response with status, headers and content. HTTPS is HTTP over TLS, which encrypts the data, verifies the server’s identity with certificates and protects messages from tampering in transit.
Ans. Use DFS with a recursion stack to detect a cycle in a directed graph. Mark each node as unvisited, visiting, or visited. During DFS, if you reach a node marked visiting, there is a cycle. If DFS finishes, mark it visited. This takes O(V + E) time and O(V) space.
Ans. A locking system in a DBMS controls concurrent access to data so transactions do not conflict and data stays consistent. A shared lock allows multiple transactions to read, while an exclusive lock allows one transaction to write. The key issue is lock granularity and protocols like two phase locking, which help ensure serialisability.
Ans. JWT authentication uses a signed JSON Web Token to prove a user’s identity without storing session state on the server. After login, the server issues a token containing claims such as user id and expiry. The client sends it in the Authorization header. The server validates the signature, expiry and claims before allowing access.
Ans. KNN is used in sentiment analysis by representing each text as a feature vector and assigning the sentiment most common among its k nearest labelled examples. Text is usually vectorised with TF-IDF, bag-of-words, or embeddings, and cosine similarity is often used because word-count vectors are high-dimensional and sparse.
Ans. Swapping is an operating system memory management technique where data, pages, or entire processes are moved between main memory and disk. It frees RAM when memory is under pressure and brings the data back when needed. The key point is that disk access is much slower than RAM, so excessive swapping hurts performance.
Ans. 1NF means atomic fields, 2NF removes partial dependency on a composite key, and 3NF removes transitive dependency. For example, a table with multiple phone numbers in one cell breaks 1NF. A course enrolment table storing student name by student ID breaks 2NF. Storing department location via department ID in employee breaks 3NF.
Ans. SSH keys are a pair of cryptographic keys used to authenticate securely to remote systems, commonly over SSH. The public key is placed on the server, while the private key stays on the client and must be protected. During login, the server verifies you hold the private key without receiving it.
Ans. Use Quickselect to find the Kth largest element by partitioning the array around a pivot and only recursing into the side that can contain the answer. Convert it to the index n minus k in sorted ascending order. Average time is O(n), worst case O(n²), with O(1) extra space.
Ans. A Java thread moves through NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING and TERMINATED states. It is NEW after creation, becomes RUNNABLE after start(), may run when scheduled, may block on locks or wait for signals or timeouts, and ends when run() completes or throws. A terminated thread cannot be restarted.
Ans. NPM packages are versioned using semantic versioning: major, minor and patch, such as 2.3.1. Dependencies are declared in package.json, often with ranges like caret or tilde. The package-lock.json file records exact resolved versions, making installs repeatable across machines and environments.
Ans. ACID describes reliable database transactions: Atomicity means all or nothing, Consistency preserves valid rules, Isolation hides concurrent transaction effects, and Durability makes committed data survive failures. CAP says a distributed system cannot fully guarantee Consistency, Availability, and Partition tolerance at once; during network partitions, it must choose consistency or availability.
Ans. A credit card payment goes from authorisation to clearing and then settlement. The merchant sends the transaction to its acquirer, through the card network, to the issuer. The issuer checks the card, funds, fraud rules and approves or declines. Later, approved transactions are batched, fees are applied, and money is settled to the merchant.
Ans. Common tree types include general trees, binary trees, binary search trees, balanced trees, heaps, tries, and B-trees. The key difference is how nodes are organised and what operation they optimise. Binary search trees support ordered lookup, heaps support priority access, tries support prefix search, and B-trees support efficient disk-based indexing.
Ans. Add the MySQL JDBC driver, import JDBC classes, create a connection using DriverManager with the MySQL URL, username and password, then create a Statement or PreparedStatement to run queries. Process the ResultSet for reads, handle exceptions, and close ResultSet, Statement and Connection, preferably with try-with-resources.
Ans. JWT is a JSON Web Token, a compact signed token used to prove a user’s identity and claims to a REST API. After login, the server issues a JWT and the client sends it in the Authorization header, usually as a Bearer token. The key detail is that the API verifies the signature and expiry before trusting it.
Ans. The Singleton pattern ensures a class has exactly one instance and provides a global access point to it. It is usually implemented with a private constructor and a static method or property returning the instance. The key detail is thread safety, especially if the instance is created lazily in a multi-threaded program.
Ans. Deadlocks in Java are avoided by ensuring threads acquire locks in a consistent global order and release them promptly. Keep synchronised sections small, avoid calling external code while holding locks, and prefer higher-level concurrency utilities. ReentrantLock with tryLock and timeouts can help recover, while thread dumps can diagnose deadlocks.
Ans. Pick a real dispute where you stayed calm, listened to both sides, and helped move the team towards a decision. Emphasise facts, respect, shared goals, and any compromise or escalation used. Interviewers listen for emotional control, fairness, communication, accountability, and proof that conflict led to progress rather than blame.
Ans. Use binary search to find the pivot, usually the smallest element, where the sorted order restarts. Compare the middle element with the rightmost element: if mid is greater, the pivot is in the right half; otherwise it is in the left half including mid. This takes O(log n) time and O(1) space.
Ans. Multithreading provides multiple threads that can run, and CPU scheduling decides which runnable thread gets CPU time and when. On a single core, scheduling interleaves threads to give concurrency. On multiple cores, it can run threads in parallel. The key detail is that schedulers usually schedule kernel-level threads, not whole processes.
Ans. Authentication verifies a user’s identity, usually by checking submitted credentials against stored user data, then issuing a session cookie or token for later requests. The key detail is that passwords should never be stored directly; store salted password hashes, use HTTPS, and protect cookies with secure, HttpOnly and same-site settings.
Ans. The main types are stack memory and heap memory. The stack stores function calls, local variables, and primitive values with short lifetimes. The heap stores objects, arrays, closures, DOM nodes, and other dynamically allocated data. In JavaScript, heap memory is managed by garbage collection when objects are no longer reachable.
Ans. Use SELECT to choose columns, FROM to name tables, WHERE to filter rows, JOIN to combine related tables, GROUP BY for aggregation, and ORDER BY for sorting. The key detail is matching joins on primary and foreign keys. Indexes on filter and join columns usually make retrieval close to logarithmic lookup rather than full table scans.
Ans. Use prefix sums and a hash map of prefix sum frequencies to count zero-sum subarrays in one pass. Start with frequency of sum 0 as 1. For each element, update the running sum; if it has appeared before, add its frequency to the answer, then increment that frequency. Time is O(n), space is O(n).
Ans. About 1.5 to 2 lakh marriages happen in Delhi each year. Take Delhi’s population as roughly 2 crore. About 1.5% to 2% of the population enters the main marriage age band each year, and roughly half form couples. That gives around 150,000 to 200,000 weddings annually, before small adjustments for remarriages or migrants.
Ans. OAuth is an authorisation framework that lets an application access a user’s resources without handling their password. I implemented OAuth 2.0 using the authorisation code flow: redirecting users to the provider, exchanging the returned code for tokens, storing refresh tokens securely, and sending access tokens on API requests.
Ans. DevOps is a culture and set of practices that bring development and operations together to deliver software faster and more reliably. Docker packages applications with their dependencies into portable containers. Kubernetes orchestrates those containers, handling deployment, scaling, service discovery, load balancing, and recovery when containers or nodes fail.
Ans. Use a relational schema with tables for students, guardians, teachers, classes, subjects, enrolments, attendance, exams, marks, fees and users. The key detail is modelling many to many relationships with junction tables, such as student enrolments in classes and teacher assignments to subjects. Add primary keys, foreign keys, unique constraints and indexes on common searches.
Ans. A website flow starts when the browser requests a page, renders the frontend, then sends API requests to the backend for data or actions. The backend receives the request, validates it, runs business logic, reads or writes the database, and returns a response, usually JSON, which the frontend uses to update the UI.
Ans. CPU scheduling is how the operating system chooses which ready process or thread runs next on the CPU. The scheduler keeps runnable tasks in scheduling queues and applies a policy such as round robin, priority, or shortest job first. The key trade-off is fairness, responsiveness, throughput, and context-switch overhead.
Ans. Make two straight cuts down through the cake at right angles, crossing in the centre, so the top is divided into four equal quarters. Then make one horizontal cut halfway up the cake, parallel to the table. That single cut splits each quarter into two equal layers, giving 4 times 2 equals 8 equal pieces.
Ans. Assuming each wire takes 60 minutes to burn, light the first wire at both ends and the second at one end. The first wire burns out in 30 minutes. At that moment, light the other end of the second wire. Its remaining burn time is 30 minutes, so burning from both ends takes 15 more minutes: 45 total.
Ans. Choose the few parameters that drive the estimate, split the problem into simple parts, assign plausible values, then multiply through and sanity check. For example, estimate daily coffee cups in London as population times coffee drinkers times cups per drinker: 9 million times 50% times 1.5 gives about 7 million cups per day.
Ans. Use orientation tests: a point lies inside or on a triangle if it is on the same side of all three directed edges. Compute the sign of cross products for edges AB, BC and CA with the point. If all signs are non-negative or all non-positive, return true. Time complexity is O(1).
Ans. Choose a real but not damaging example, such as delaying feedback, taking on too much work, or not asking for help early. Emphasise your judgement at the time, what went wrong, and the specific lesson you applied later. Interviewers listen for ownership, self-awareness, maturity, and evidence that regret led to better decisions.
Ans. Manage npm package versions through package.json and the lock file, using semantic version ranges for allowed updates and package-lock.json for exact installed versions. Check changes with npm outdated, update with npm update, or install a specific version with npm install package@version. Commit the lock file and test after upgrades.
Ans. A Trie is a tree data structure used to store strings by sharing common prefixes. Each node represents a character, and paths from the root form words. Common operations are insert, search, and prefix search. Each takes O(L) time, where L is the word length, but Tries can use significant memory.
Ans. Use a CREATE TABLE statement with column definitions followed by constraints such as primary key, foreign key, unique, not null, check and default. The database stores this as schema metadata, not an application data structure. Creating the table is generally constant time, excluding validation or storage engine overhead.
Ans. Cookies are small name value pieces of data that a server asks a browser to store and send back on later requests. For sessions, the server usually stores session data server side and puts only a session ID in the cookie, letting it recognise the user across stateless HTTP requests. Secure, HttpOnly and SameSite flags reduce risk.
Ans. Graphs are used to model relationships and connections, such as social networks, web pages, computer networks, maps, dependency graphs, and recommendation systems. The key idea is that vertices represent entities and edges represent relationships, allowing algorithms like BFS, DFS, shortest path, and topological sort to solve real problems efficiently.
Ans. SQL databases are best for structured data, strong consistency, complex queries and transactions, while NoSQL databases suit flexible schemas, high scale, fast writes and distributed systems. Use SQL for banking, orders or reporting where relationships matter. Use NoSQL for logs, feeds, caching, documents or rapidly changing data models.
Ans. About 1.5 million people. Delhi has roughly 20 million residents, perhaps 12 million adults. If around 25% of adults live in car-owning households, that is 3 million potential car users. Assuming about half are regular drivers and drive themselves rather than using chauffeurs, the estimate is roughly 1.5 million.
Ans. Choose a case where you quickly understood the business context, identified the core problem, and made a structured recommendation. Emphasise your reasoning, trade-offs, data used, and collaboration with stakeholders. Interviewers listen for commercial awareness, clear thinking, prioritisation, communication under pressure, and whether your conclusion is practical rather than just theoretically correct.
Ans. I would handle it by making stock reservation an atomic operation in the database or inventory service. Each order attempts to decrement available stock only if stock is still positive, using a transaction with row locking or optimistic version checks. Failed updates mean the item is sold out, preventing overselling under concurrent requests.
Ans. I would make request handling non-blocking, keep shared state minimal, and use concurrency controls so slow I/O does not block other users. The key detail is to protect bottlenecks: use connection pooling, caching, queues for expensive background work, rate limits, and horizontal scaling with stateless workers behind a load balancer.
Ans. The probability is 1/3, assuming two children and “one is a girl” means at least one is a girl. List equally likely possibilities: BB, BG, GB, GG. Remove BB because it has no girl. Of the remaining three cases, only GG has both girls. If a specific child is known to be a girl, it becomes 1/2.
Ans. Reverse a linked list by iterating through it and changing each node’s next pointer to point to the previous node. Keep three pointers: previous, current, and next, so you do not lose the rest of the list. At the end, previous is the new head. Time complexity is O(n), space complexity is O(1).
Ans. Use Floyd’s tortoise and hare algorithm: keep two pointers, one moving one node at a time and the other moving two nodes at a time. If they ever meet, there is a cycle. If the fast pointer reaches null, there is no cycle. It uses constant extra space and runs in O(n) time.
Ans. List the prime numbers in order and compare the given series with that list. Check whether it uses consecutive primes or skips by a fixed pattern, such as every second prime or increasing gaps. Once the rule is clear, continue the same pattern and choose the next prime in that position.
Ans. 75. A number that is a multiple of both 3 and 4 must be a multiple of their LCM, 12. Count the 3-digit multiples of 12: from 100 to 999. This is floor(999/12) minus floor(99/12), which is 83 minus 8, giving 75.
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Candidate interviews most often cover CS fundamentals (55%) and DSA (27%).
Candidate interviews show an average of 3.4 rounds per experience, with a typical sequence of Online test → Technical → Managerial. Individual interview paths can vary.
Among questions with a recorded difficulty, the mix is easy 53%, medium 45%, hard 2%.
